There's no such thing as a "dofollow" tag... "Dofollow" is the default for pages and links, thus no need to specify if you want the link(s) followed... only if you don't.

@bestofshayari: The tag does nothing to affect traffic at all, it simply tells Google that you don't "vouch" for the linked site and prevents passage of "link juice" to that site (meaning the site gets little or no SEO benefit from the link).

Note: It does NOT affect the passage of "link juice" to the other links on the page, the juice for the no-followed link(s) is simply "flushed away".

To answer your question: You would use it to indicate to Google that the link is not intended to be used as a "vote" for the linked page/site.